Methods for drying electrical apparatus



March 19, 1968 D. F. WINTER 3,

METHODS FOR DRYING ELECTRICAL APPARATUS Filed Aug. 26, 1966 2 Sheets-Shem 1 F IG.I.

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March 19, 1968 D. F. WINTER 3,

METHODS FOR DRYING ELECTRICAL APPARATUS Filed Aug. 26, 1966 2 Sheets-Sheet DC. TIMING POWER UNIT SOURCE DC. POWER SOURCE SPARKOVER VOLTAGE PRESSURE Patented Mar. 19, 1968 3,373,500 METHODS FOR DRYING ELECTRICAL APPARATUS David F. Winter, Kirkwood, Mo, assignor to Central Transformer Corporation, Pine Bluff, Ark, a corporation of Arkansas Filed Aug. 26, 1966, Ser. No. 575,302 Claims. (Cl. 34-1) This invention relates to methods for drying electrical apparatus, and more particularly to methods for drying such apparatus having core-and-coil units including moisture-absorbing insulation.

In the manufacture of electrical apparatus such as transformers, it is necessary to enclose certain compoents, e.g., the core-and-coil unit, in a sealed housing and remove essentially all moisture in order to insure high dielectric and insulation characteristics of the completed apparatus. Much of the insulation used in the fabrication of such units is moisture absorbing and is usually cellulosic in nature, for example, press-board, paper and cotton cloth, etc. As the windings, insulation and core laminations are assembled under plant atmosphere conditions in which the ambient humidity levels are such that the insulation will contain several percent or more of moisture, the core-and-coil unit must be tanked or enclosed and all significant moisture removed before the transformer may be put into service. The usual practice followed is to preheat the core-and-coil unit after assembly and immediately prior to placement in the tank and sealing. A vacuum is then drawn on the sealed tanked unit to remove the moisture. However, the effectiveness of moisture removal is a function of the temperature of the enclosed unit to which the high vacuum is applied. As a substantial period of time is required, particularly with large core-and-coil units, the unit will cool down even though preheated to relatively high temperatures prior to installation in the tank. This cooling inhibits the complete removal of moisture and greatly prolongs the drying process. The drying of large power transformers using this conventional process requires about a week.

In order to reduce this costly and time-consuming drying process, it has also been proposed that hot transformer oil or other liquid dielectric be circulated or impinged on the core-and-coil unit which is being dried under vacuum. However, this process is also quite expensive and entails various difficulties and disadvantages in moisture removal. The possible use of external heaters and similar methods of supplying heat to the encased bulky core-and-coil unit during the evacuation process also presents problems which have not been solved practically. Electrically energizing the windings to generate heat by applying AC. power to one winding while another is shorted is found impractical because during the evacuation-moisture removal period the dielectric strength is so reduced that even or so of normal rated impedance voltage cannot be applied to the unit Without risking failure of the unit by voltage breakdown.

Among the several objects of this invention may be noted the provision of methods of drying electrical apparatus having a core-and-coil unit including moistureabsorbing insulation, which methods greatly reduce the time of drying and avoid any danger of electrical breakdown or failure during this accelerated or shortened drying period; the provision of such methods in which the temperature of the core-and-coil unit and the individual windings thereof can be closely controlled to increase the amount and efficacy of water removal; and the provision of methods of the type described which significantly reduce manufacturing costs of the apparatus and markedly improve delivery schedules. Other objects and features will be in part apparent and in part pointed out hereinafter.

In accordance with the present invention, methods are provided for effective and rapid drying of electrical apparatus having a core-and-coil unit including moistureabsorbing insulation and at least one outer winding by generating a desired amount of heat in the winding, viz., that amount of heat necessary to prevent a substantial drop in the temperature of the core-and-coil unit while it is under partial vacuum in a sealed casing. These methods include the step of energizing at least the outer winding, while the unit is under partial vacuum, from a DC. power source at a voltage level which is substantially less than that of an A.C. voltage which would be re quired to produce substantially the same current and amount of heat. Thus, the desired heating effect is provided by the thus D.C.-energized winding at greatly reduced dielectric stresses. These methods also include applying sequential D.C. power pulses of the same or alternately opposite polarity to one winding while another is shorted or interconnected with the other windings, thereby to supply the amount of heat desired during drying in a partial vacuum and at greatly reduced dielectric stresses. The duration or period of the pulses is at least ten times greater than that of A0. of a frequency for which the core-and-coil unit is designed to operate. Preferably the DC. pulse periods are such that saturation of the core will be approached but not attained during each pulse.

The invention accordingly comprises the methods hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawings, in which several of various possible embodiments of the invention are illus trated,

FIG. 1 illustrates a power transformer (with sections broken away to show the core-and-coil unit) which is be ing dried in accordance with an exemplary method of the present invention;

FIGS. 2-4 illustrate alternative methods of this invention in which a transformer core-andcoil unit is energized to provide heating during drying under a partial vacuum and at reduced dielectric stresses; and

FIG. 5 graphically illustrates a typical relationship between internal tank pressure and spark-over voltage during the evacuation and drying of a transformer.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Referring now to the drawings, a transformer is generally indicated at reference numeral 1 and includes a tank or housing 3 in which is mounted a core-and-coil unit 5. This latter unit comprises a laminated steel core 7 and at least one high voltage winding 9 and at least one low voltage winding 11. The terminals of the windings are interconnected in the customary fashion to high voltage bushings 13 'and low voltage bushings 15. Necessary conventional insulation, such as pressboard members 1.7 (FIG. 2) of cellulosic material and the usual paper tape and other insulation material used in the construction of transformers, is inherently moisture absorbing. Thus the core laminations, insulation member's, windings, etc., as they are assembled on the shop floor in the ambient plant atmosphere, must be dried before the core-and-coil unit is finally sealed in its tank and the tank substantially filled with any of the conventional dielectric fluids. In order to dry this core-and-coil unit and eliminate the moisture from within the tank and unit 5, a trap 19 is temporarily secured to the tank by a duct 21 which provides gaseous communication between the tank interior and a vacuum source (such as a vacuum pump) by means of a vacuum line 23.

Connected electrically across the high voltage bushings 13 (only one of which is specifically shown) is a DC. power source 25, a series switch 27 and a capacitor 29. Core-and-coil unit preferably preheated in a large oven to about 90l00 C., is lowered into tank 3 which is temporarily sealed after making internal connections between the windings and the bushings. The vacuum source is immediately connected and evacuation of the tank is initiated. In order to expeditiously and effectively reduce the internal moisture content to the insignificant necessary level, the temperature of core-and-coil unit 5 is desirably maintained at the 90-100 C. level during this evacuation-drying process inasmuch as the moisture removal is dependent on the vacuum level and the relative vapor pressure of the moisture, the latter being a function of the temperature. The core-and-coil unit 5, however, will radiate heat to the tank Walls which dissipate heat rapidly from their large surface areas. Thus the outer surface of unit 5 will lose heat and cool unless the temperature thereof can be maintained. External heating of the tank 3 is not only impractical but is not economical. Applying AC. to the outer high voltage winding 9 with the low voltage winding 11 shorted will generate heat, but in order to cause suflicient current flow (e.g., 40% of the rated current) to maintain the outer winding at the desired temperature, at least about %70% of the normal impedance voltage would have to be applied. The dielectric strength of the moisture-containing partial vacuum is not sulficient to withstand potentials of even such values without failure.

In accordance with the present invention, a current of substantially this same or predetermined value is made to flow in the outer winding by energizing winding 9 from DC. power source 25 by positioning switch 27 in the position illustrated in FIG. 1. The D.C. potential level required to develop this current is substantially less than that of the AC. voltage level necessary to produce the same current, and thus the desired amount of heat is generated by the thus D.C.-energized winding and at greatly reduced dielectric stresses. For example, it was determined that 28,000 v. A.C. (at a line frequency of 60 c.p.s.) would have to be applied across the outside high voltage windings of a 230 kv. Y-connected power transformer to produce a current of suificient value to maintain the temperature of these outside windings at 90100 C. while in the tank 3. However, in the practice of this invention, only 70l00 v. D.C. need to be applied to these windings to produce substantially the same current fiow and heating effect. In a comparison of the conventional drying prac tices (preheated core-and-coil unit in the sealed tank without energization while evacuating) with the methods of the present invention, it was found that the same quantity of moisture which was removed from unit 5 in seven days was removed in only thirty-six hours in accordance with the present invention.

It is important to note that the reduction in dielectric stresses is particularly important in the course of reducing the internal tank pressure from an ambient atmospheric level, immediately following scaling, to the high vacuum condition cXisting at the completion of the drying step. The voltage level or sparkover potential which will effect dielectric breakdown in a moisture-containing atmosphere is not a linear function of pressure, i.e., lowering the pressure does not proportionately increase the sparkover potential. At some partial vacuum level attained during this process there is a minimum sparkover potential while under higher and lower vacuum conditions the unit Will withstand a higher applied voltage without sparkover or failure. This is illustrated in FIG. 5.

The progress and completion of the moisture removal step may be determined in any of several usual ways known to those skilled in the art, e.g., periodically inspecting optional cold trap 19 to see if water is still being condensed and collected, or monitoring the internal temperature and pressure from which data the internal moisture content can be easily calculated. The internal winding temperatures may be conveniently sensed by measuring the resistance of the windings. During the evacuationdrying step the DC. energized winding will efiectively serve as a heat radiation shield assuming the unit 5 was preheated before being tanked, i.e., the heat supplied by the IR losses of the D.C.-energized outer high voltage Winding 9 prevents or shields the inner portions of unit 5 from loss of heat. It will be understood that conventional thermostatic controls may be used during the drying step, and switch 27 may be operated in response to the internal temperature of unit 5 sensed at a preselected location to thus maintain the temperature at the desired level. After this drying step, the interior of transformer 1 may be purged with dry nitrogen and then have the transformer oil introduced, after the purging nitrogen has been evacuated.

In some instances it may be desirable to reduce the remanence of the core and tank after the drying operation. An optional adjustable low voltage A.C. supply source 31 and another contact on switch 27 are provided for this purpose. By throwing switch 27 to disconnect D.C. source 25- and connect AC. source 31 to winding 9 and applying an AC. potential thereacross which is initially sufiicient to effect a current flow somewhat greater (e.g., about 20%) than the DC. current flow during drying, and thereafter reducing or diminishing the amplitude of the A.C. applied thus to diminish the A.C. current, the remanence of the core and tank will be reduced. An alternate way of reducing the remanence is to apply an opposite-polarity DC. voltage to winding 9 for a preselected period of time.

Another embodiment of this invention, as indicated in FIG. 2, involves D.C. energization of another Winding of the core-and-coil unit 5 by a DC. power source 2&2. Thus two switches 27a and 27b are used and additional heat may be generated at reduced dielectric stresses within encased unit 5 during evacuation and drying. In this instance the inner low voltage winding 11 is so energize-d. It will be noted that if unit 5 has additional windings they may also be separately energized by DC. or may be interconnected to other windings and commonly energized therewith. Also, the outermost winding preferably energized need not be the high voltage winding.

A further embodiment of this invention is indicated in FIG. 3. In this alternative method the DC. applied to winding 9 is pulsed D.C., and the low voltage winding 11 is shorted as indicated. Assuming that the predetermined current ilow value in winding 9 has been ascertained empirically or by calculation, a series of DC. pulses are applied to windin g 9 having a voltage suiiicient to develop an average current flow substantially equal to this predetermined current value. The pulses are developed, as shown schematically, by a switch 270 which is intermittently closed and reopened by any conventional timing unit 33. The duration of each pulse is quite long relative to that of AC. of a frequency approximately equal to that at Which the transformer l is designed to operate. The pulse period is at least ten times as long as that of AC. of the particular line frequency, i.e., at least one or more orders of magnitude greater. Preferably, this pulse period for a typical 60 c.p.s. power transformer is at least about 1-10 seconds, the duration being suiiicient for the core to approach but not attain a saturation condition. Switch 270 is then reopened for a brief period of time, e.g., one-tenth that of the pulse, and then reclosed. Such pulsed D.C. energization of winding 9 induces current flow in winding 11, the desired amount of heat being generated in unit 5 at a maximum voltage which is only a fraction of the AC. voltage which would have to be applied to eifect substantially the same current flow. The ratio of such AC. potential to the much smaller DC. voltage required to effect the same current iiow and heat generation is approximately the same as the ratio of the period of the DC. pulse to that of AC. of the frequency applied.

A diode 35 is shunt-connected across winding 9 as a bleeder rectifier to provide a low resistance discharge path for the collapsing field of winding 9 when switch 27a is opened and thereby prevents arcing at switch 27c. It will be understood that a capacitor or other equivalent means will also function to prevent such undesirable arci still further embodiment of the present invention is described with reference to FIG. 4, in which a reversing switch 27d interconnects DC. power source 25 to winding 9. Following the procedure described above, relatively low voltage D.C. pulses of relatively long duration but of alternately opposite polarity are supplied at a voltage level sulficient to induce the predetermined amount of current to generate the desired amount of heat in the unit 5 to compensate for the heat loss from the sealed tank 3 during the evacuation and drying process. The period of each pulse will be, as in the preceding exemplary method described above, at least one or more orders of magnitude greater than that of AC. at line frequency, and will be typically about a 1-10 second positive pulse followed by a one second off period followed by a 1-10 second negative pulse, etc. Again a timing drive unit may be used mechanically to move switch 27d between its two energizing positions. Also, it will be understood that solid-state devices and timing circuitry well known to those skilled in this art may be used as a full equivalent of mechanical timers and switches.

The low voltage winding is shown as having two winding sections 11a and 1112 which are loop-connected. However, it is to be understood that such winding sections and other windings of multiphase transformers, for example, may be interconnected together in series or parallel or any desired combination thereof for heating by the DC. voltage source, continuous or pulsed, unidirectional or alternate polarity pulses, to obtain the predetermined current flow and generate the desired amount of heat in the tanked core-and-coil unit at greatly reduced dielectric stresses while at partial vacuum during the accelerated drying resulting from the methods of the present invention.

It will also be noted that instead of using a large oven to preheat the core-and-coil unit before tanking, the various windings may be energized from a DC. source or sources, and thereby develop the heat internally and economically.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method of drying electrical apparatus having a core-and-coil unit including moisture-absorbing insulation and at least one outer Winding which will generate a desired amount of heat when energized at an AC. voltage level suflicient to produce a predetermined value of current; the step of energizing said winding, while said core-and-coil unit is under partial vacuum, from a DC. power source at a voltage substantially less than that of said AC. voltage level to produce a current of said predetermined value whereby substantially the desired amount of heat is generated by said winding at greatly reduced dielectric stresses.

2. In a method as set forth in claim 1, said method further including subsequently applying a diminishing amplitude AC. voltage to said winding to reduce the remanence of the core.

3. In a method as set forth in claim 1, said method further including subsequently applying a DC. voltage of an opposite polarity to said winding at a level and for a preselected period of time to reduce the remanence of the core.

4. In a method as set forth in claim 1 and in which there is a second Winding on the core-and-coil unit; the further step of concurrently energizing said second winding from a DC. power source at a voltage substantially less than that of said AC. voltage level whereby additional heat is generated by said second winding also at reduced dielectric stresses.

5. In a method of drying electrical apparatus having a core-and-coil unit including moisture-absorbing insulation and at least first and second windings which will generate a desired amount of heat when the second winding is shorted and the first winding is energized by AC. of a frequency approximately equal to that at which the unit is designed to operate and at an AC. voltage level sufficient to produce a predetermined value of current; the step of energizing said first winding, while said core-andcoil unit is under partial vacuum, with sequential pulses of DC. power at a voltage substantially less than that of said AC. voltage level and of a period which is at least ten times longer than that of said A.C. frequency to produce a current of said predetermined value whereby substantially the desired amount of heat is generated by said windings at greatly reduced dielectric stresses.

6. In a method as set forth in claim 5, the DC voltage level and the pulse period being such that the core approaches saturation during each pulse.

7. In a method as set forth in claim 5, the DC. pulses being alternately of opposite polarity.

8. In a method as set forth in claim 5 and in which the core-and-coil unit includes another winding; the further step of energizing this latter winding to generate additional heat by shorting this winding.

9. In a method as set forth in claim 5 and in which the core-and-coil unit includes another winding; the further step of energizing this latter winding to generate additional heat by interconnecting this latter winding with said second winding.

10. In a method as set forth in claim 5, the further step of discharging said first winding through a low resistance path after each D.C. pulse.

References Cited UNITED STATES PATENTS 1,871,269 8/1932 Hobrock 34-1 1,941,913 1/1934 Riley. 3,299,524 1/1967 Jacobs 34-1 FOREIGN PATENTS 786,220 1 1/ 195 7 Great Britain.

OTHER REFERENCES Electrical InsulationIts Application to Shipboard Electrical Equipment, by Graham Lee Moses. Published by McGraw-Hill Book Co., New York, N.Y., 1951, pp. 132, 133, 134, and are of interest.

CHARLES J. MYHRE, Primary Examiner. J. J. CAMBY, Assistant Examiner. 

1. IN A METHOD OF DRYING ELECTRICAL APPARATUS HAVING A CORE-AND-COIL UNIT INCLUDING MOISTURE-ABSORBING INSULATION AND AT LEAST ONE OUTER WINDING WHICH WILL GENERATE A DESIRED AMOUNT OF HEAT WHEN ENERGIZED AT AN A.C. VOLTAGE LEVEL SUFFICIENT TO PRODUCE A PREDETERMINED VALUE OF CURRENT; THE STEP OF ENERGIZING SAID WINDING, WHILE SAID CORE-AND-COIL UNIT IS UNDER PARTIAL VACUUM, FROM A D.C. POWER SOURCE AT A VOLTAGE SUBSTANTIALLY LESS THAN THAT OF SAID A.C. VOLTAGE LEVEL TO PRODUCE A CURRENT OF SAID PREDETERMINED VALUE WHEREBY SUBSTANTIALLY THE DESIRED AMOUNT OF HEAT IS GENERATED BY SAID WINDING AT GREATLY REDUCED DIELECTRIC STRESSES. 